The holiday this time is to be in the Black Mountains. Allison is going to paint and stay at a chalet, Bill will be working, but visiting and Hans will take the children on a pony trek for a couple of days.

As they approach their destination a helicopter swoops overhead. Far away in the valley a young man called Alexai flees for his life as the pilot of the helicopter searches the ground below.

Next day the kids and Hans set off. Later they picnic and Hans recounts the legend of the Mountain of Kings. Lucy-Ann has a premonition about it but the others are intrigued. After a night at their campsite, the kids awake to a thick mist. They set off in reduced visibility. Hans thinks his compass is broken.

At their next camp the horses are scared off by wolf like creature. Hans manages to vault onto one of the ponies and sets off in pursuit. Further on he is knocked from the saddle by a branch. He falls to the ground, unconscious. The kids wander around in the mist. Their supplies were packed on the ponies so they must hunt for food.

Bill proposes to Allison and she accepts, but her happiness is clouded as the children fail to turn up when they should. A search is impossible until morning, the mist is too thick.

During the night Philip befriends a group of Alsatians, removing a thorn from one of their paws.

Next day a search party is organised to look for the kids. They have spent a chilly but optimistic night and in the morning Lucy-Ann is sent for water. She discovers Alexai who is as startled as she is. When she brings the other children to see, he is gone. The Mountain rumbles ominously. While wandering in the foothills, they see Alexai being pursued by dogs. Philip goes to his rescue and ends up being caught with Alexai by two guards with guns. The others decide to follow but fall into a snow hole.

Hans is recovered by the rescue team but is in a coma.

Alexai and Philip are ushered into the interior of the Mountain and locked up. In broken English, Alexai explains that he is to be used as a human guinea pig to test a flying machine designed by the King of the Mountain, Doctor Grunwald.

Hans regains consciousness and tells Bill and Allison that the children were interested in exploring the Mountain of Kings. They set off.

Jack, Dinah and Lucy-Ann escape from the snow hole and find their way into the Mountain of Kings with help from Kiki. They find the dungeon where Alexai and Philip are kept and decide to find out what is going on. The children take a wrong turning and end up overlooking the anti-gravity cavern. A blinding light, a tremendous noise and they float towards the ceiling. They manage to drag themselves to safety.

Bill and Allison are in their car when the Mountain rumbles. They think it is an earthquake. Bill sees a helicopter landing and decides to interrogate the pilot.

The guards have discovered that Philip is not with Alexai and set off the alarm. While trying to escape the kids are recaptured. At this point they meet Maddern, Erlich and Grunwald. They discover that they are to be used to test the anti-gravity machine and that Maddern is going to exploit the invention to make millions from the oil industry by preventing the machine’s use. Grunwald hopes it will lead to a pollution free environment.

Back in their cell, Philip decides to inform Grunwald of Maddern’s intentions and climbs out of his window to Grunwald’s room below. Grunwald is horrified when he hears Philip’s news but the power hungry Maddern insists that the test will take place next morning.

Philip is strapped into the anti-gravity suit. On the helicopter pad at the top of the Mountain the kids huddle together fearfully as Philip is bundled into a waiting helicopter. It takes off. Philip turns to the pilot. It is Bill. Maddern realising the helicopter pilot is not obeying his instructions gets the guards to open fire. The helicopter lands and Allison rushes up to hug Philip.

Special Forces swarm like flies over the Mountain. Pandemonium. Bill lands his helicopter on the pad and rescues the rest of the kids. Grunwald, knowing all is lost heads for the anti-gravity chamber to sabotage his work. Maddern tries to stop him but is distracted by Kiki. The Mountain begins to break up and finally implodes.

While Bill and the kids are searching for Kiki, they encounter Maddern. He attaches an anti-gravity suit to himself and leaps into space and ultimately to his death as the device fails.

Out of the rubble Grunwald emerges still figuring out what went wrong with his invention.

Back home some weeks later Philip receives a package. It contains an anti-gravity suit. Grunwald is alive and well and continuing his work in Argentina.